Allow to upload a version with the same number as rejected versions (bug 750909)

Fixes [bug 750909](https://bugzilla.mozilla.org/show_bug.cgi?id=750909)
I'm not sure what you are comparing here.

`self.version` is the disabled version.
`Version.objects.first()` is.. the same version ?

I'd do something like that instead:
```
self.version.reload()
version = Version.objects.latest()
ok_(version.pk != self.version.pk)
```
...